1. Field of the Invention
This invention relates to a micro-positioning tool post for precision positioning in ultra-precision machining.
2. Prior Art Statement
For use in ultra-precision machining there is already known a micro-positioning post which uses a piezoelectric actuator for displacing a tool supported on a flexible support.
In this type of micro-positioning post it is necessary to conduct closed-loop control by feeding back the tool displacement to the voltage applied to the piezoelectric actuator. The main reasons for this are (i) that it is not possible to carry out precision positioning of the tool without eliminating the hysteresis characteristics of the piezoelectric device, (ii) that it is necessary to conduct closed-loop control or the like in order to compensate for the insufficient stiffness of the tool stage in the thrust direction, and (iii) that there is a need to suppress the sharp resonance characteristics at the resonance frequency determined by the mass of the spring and moving member.
As shown in FIG. 7, the typical closed-loop control system conventionally employed for coping with these problems includes a notch filter for canceling the resonance of the tool post and in this way smoothing the frequency characteristics. The loop also generally includes an integrator for increasing the loop gain in the low frequency region.
While this type of control system improves the static characteristics (including the hysteresis characteristics), the dynamic characteristics (expressed in terms of the frequency characteristics), and the static stiffness (for minimizing positional change caused by static forces), it does not provide any improvement in the dynamic stiffness required for minimizing positional change caused by external forces (disturbance), which include a variable component.